The breeding and development of crops has been ongoing across the last 1000 years. The pace of this development in soybeans, as an animal foodstuff and as an oil seed has dramatically increased in the last one hundred years. Planned programs of plant breeding have increased the growth, yield and environmental hardiness of the soybean germplasm. Due to the sexual reproduction traits of the soybean the plant is basically self-pollinating. A self-pollinating plant permits pollen from one flower to be transferred to the same or another flower of the same plant. Cross-pollination occurs when the flower is pollinated with pollen from a different plant. This is a rare occurrence in nature.
Thus the growth and development of new soybean germplasm requires intervention by the breeder in the pollination of the soybean. The breeders' selections of methods of intervening in the pollination depend on the type of trait that is being selected. Soybeans are developed for a number of different types of traits morphological (form and structure), phenotypical, for growth habit, daylength temperature requirements to initiate floral or reproductive development yield. The genetic complexity of the trait drives the breeding method. Backcross breeding is employed when the cultivar that is being bred has a fairly full profile of desirable traits, but lack one or two traits that are highly inheritable. Backcrossing is often employed to move disease resistance, insect resistance and transgenes (hereinafter DNA which are introduced into the original ancestor germplasm by a transformation method) into other varieties.
When the variety is being employed to develop a new variety or an improved variety the selection methods include pedigrees, recurrent, modified and mass selection and backcrossing. Each of these selection techniques is employed with the efficiency of the breeding procedure as the driver. The breeding procedure requires a continuous evaluation of the success of the breeding program. The success is measured by yield increase, commercial appeal and environmental adaptability of the developed germplasm.
New varieties must be tested thoroughly to compare the development with the commercially available soybeans. This testing usually requires at least two years and up to six years of comparisons with other commercial soybeans. Varieties that lack the entire desirable package of traits can be used as parents in new populations for further selection. The breeding and associated testing process is 8 to 12 years' progression toward a new variety. Thousands of lines are produced and limited lines are selected in each step of the process. Thus the breeding system is like a funnel with numerous lines and selections in the first few years and fewer and fewer lines in the middle years until one line is selected for the final development testing.
The selected line or variety will be evaluated for it's the growth and development and yield. These traits of a soybean are a result of the varieties genetic potential interacting with its environment. All varieties have a maximum yield potential that is predetermined by its genetics. This hypothetical potential for yield is only obtained when the environmental conditions are perfect. Since prefect growth conditions do not exist field experimentation is necessary to provide the environmental influence and to measure its effect on the development and yield of the soybean. The breeder attempts to select for good soybean yield potential under a number of different environmental conditions.
Selecting for good soybean yield potential under a number of different environmental conditions is a process that requires planning, analysis of data in a number of seasons. Identification of the varieties that carry a superior combination of traits that provides this consistent yield potential is a complex science. Other plant traits, unusual weather patterns, diseases, and insect damage often mask the genotypic traits. One widely employed method of identifying a superior plant is to observe its performance relative to commercial and experimental plants in replicated studies. These types of studies give more certainty to the genetic potential and value of the plant.
The goal of the soybean plant breeder is to produce unique and new soybeans and progeny of the soybeans. To accomplish this the plant breeder painstakingly crosses two or more varieties or germplasm. Then the results of this cross is repeatedly selfed or backcrossed to produce new genetic patterns. Additionally, the breeder can introduce mutations into the genetic material. These can alter herbicide resistance, fatty acid compositions, and amino acid compositions of the seeds and the like. Fortunately, through transformation in combination with breeding the plant breeder can alter or introduce some genetic alleles into the breeding material.
These capabilities are widening the potential uses and markets for the various products and by-products of oil seed plants such as soybean. One of the products of soybeans is the oil of the seed. Soybean oil is employed in a number of retail products. Soybean meal is also used in food and animal feedstuffs. The traits a breeder selects for can be driven by the ultimate goals of the end user of the product. Thus if the goals of the end user is to produce an oil with a high level of oleic acid and a lower level of linoleic acid then the breeder may drive the genetics toward levels of fatty acids and accept some lesser yield potentials or other less desirable agronomic traits.
Regardless of the market characteristics of the plant most breeding proceeds along a similar path on a yearly basis. The breeder annually selects the germplasm to advance on into further development. This germplasm is grown in different locations at different altitudes, in different climates and subjected to different soil conditions. Based on the datum collected from these tests individual plants are selected during the end of the growing season. Due to the number of genes within each chromosome millions of genetic combinations exist in the breeders' experimental soybean material. This genetic diversity is so vast that a breeder cannot produce the same two cultivars twice using the exact same material. Thus the developing a single variety of useful commercial soybean germplasm is highly unpredictable, and requires intensive research.
The development of new soybeans comes through breeding techniques such as recurrent selection, mass selections, backcrossing, single seed descent and multiple seed procedure that is used to save labor costs. Other breeding methods are taught in several soybean textbooks.
The development of soybean cultivars most often requires the development of hybrid crosses (some exceptions being initial development of mutants directly through the use of the mutating agent or transformants made directly through transformation methods) and the selection of progeny therefrom. Hybrids can be achieved by manual manipulation of the sexual organs of the soybean or by the use of male sterility systems. The breeder attempts to identify true hybrids by a readily identifiable trait. These hybrids are then selected and repeatedly selfed and selected to form new homozygous lines from the heterozygous hybrids.
Outcrossing to a number of different parents creates breeding populations of fairly heterozygous populations. These populations are produced and used in pedigree breeding and recurrent selection. Pedigree breeding is commonly used with two parents which possess favorable, complementary traits. The parents are crossed to form a F1 hybrid. The progeny of the F1 hybrid is selected from this the best individuals F2 are selected; this selection process is repeated in the F3 and F4 generations. The inbreeding is carried forward and at F5-F7 the best lines are selected and tested in the development stage for potential usefulness.
Mass and recurrent selection can be used to improve populations. Several parents are intercrossed and plants are selected based on selected characteristics like superiority or excellent progeny.
In backcross breeding a genetic allele or loci is transferred into a desirable homozygous recurrent parent. The trait is in the donor parent and is tracked into the recurrent parent. The resultant plant is like the recurrent parent with the new desired allele or loci.
The single-seed descent method involves use of a segregating plant population for harvest of one seed per plant. Each seed sample is planted and the next generation is formed. When the F2 lines are advanced to F6 each plant will be derived from a different F2. The population will decline due to failure of some seeds, so not all F2 plants will be represented in the progeny.
Soybean Glycine max (L) is an important oil seed crop and a valuable field crop.